Method for robust crosstalk precoder training in channels with impulse noise

ABSTRACT

An apparatus comprising a first transceiver at a central office (CO) coupled to a second transceiver at a customer premise equipment (CPE) via a digital subscriber line (DSL), a crosstalk precoder coupled to the first transceiver at the CO, and a vectoring control entity (VCE) coupled to the transceiver via a feedback channel and to the crosstalk precoder, wherein the second transceiver comprises a noise monitor configured to detect non-crosstalk noise in a downstream signal from the CO to the CPE, and wherein the first transceiver is configured to receive a predefined special feedback signal from the second transceiver that indicates whether non-crosstalk noise is detected in the downstream signal instead of a measured error value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/830,960 filed Jul. 6, 2010 by Raphael Jean Cendrillon, etal. and entitled “Method for Robust Crosstalk Precoder Training inChannels with Impulse Noise,” which claims priority to U.S. ProvisionalPatent Application No. 61/224,738 filed Jul. 10, 2009 by Raphael JeanCendrillon, et al. and entitled, “Method for Robust Crosstalk PrecoderTraining in Channels with Impulse Noise,” both of which are incorporatedherein by reference as if reproduced in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Digital subscriber line (DSL) technologies can provide large bandwidthfor digital communications over existing subscriber lines. Whentransmitting data over the subscriber lines, crosstalk interference canoccur between the transmitted signals over adjacent twisted-pair phonelines, for example in a same or nearby bundle of lines. Crosstalkintroduces noise in DSL systems and reduces the data-rates that can beachieved in the DSL systems. Thus, crosstalk can significantly limit theperformance of DSL technologies that use higher frequency bands, such asvery high bit rate DSL 2 (VDSL2). Crosstalk can be canceled or reducedby joint processing or precoding of downstream signals in multiplesubscriber lines that may be bundled, e.g. in a binder, at the networkend. Crosstalk precoding is a technique in which signals from a set ofsignals at the network central office (CO) are pre-distorted prior totransmission through the binder. A pre-distortion filter or ‘precodingmatrix’ is used to pre-distort the signals, and thus cancel crosstalkthat occurs between the lines in the binder. The signals may then arriveat the receivers at different customer sites substantially free ofcrosstalk, thereby achieving significantly higher data-rates.

A crosstalk precoder can be used in a modem, e.g. at the CO, toeliminate or reduce crosstalk in the subscriber lines. The crosstalkprecoder uses precoding coefficients, e.g. in a precoding matrix, tomodify the signals in the lines and transmits the pre-distorted signalsdownstream from the CO to a plurality of customer premise equipments(CPEs). The introduced pre-distortions in the signals substantiallycancel the crosstalk in the signals that are received by the CPEs. Thecrosstalk precoder is trained or initialized using feedback signals fromthe CPEs, which indicate the errors in the received signals at the CPEs.To train the crosstalk precoder, a VDSL transceiver office unit (VTU-O)at the CO sends a sequence of pilot symbols downstream to a VDSLtransceiver remote unit (VTU-R) at a CPE, which returns correspondingerror feedback signals to a Vectoring Control Entity (VCE) coupled tothe VTU-O and the crosstalk precoder. The error feedback signals fromthe CPEs are then used to update the precoding matrix coefficients andthus adjust the pre-distorted signals until reaching convergence.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising afirst transceiver at a CO coupled to a second transceiver at a CPE via aDSL, a crosstalk precoder coupled to the first transceiver at the CO,and a VCE coupled to the transceiver via a feedback channel and to thecrosstalk precoder, wherein the second transceiver comprises a noisemonitor configured to detect non-crosstalk noise in a downstream signalfrom the CO to the CPE, and wherein the first transceiver is configuredto receive a predefined special feedback signal from the secondtransceiver that indicates whether non-crosstalk noise is detected inthe downstream signal instead of a measured error value.

In another embodiment, the disclosure includes a network componentcomprising at least one processor coupled to a memory and configured toreceive a downstream DSL signal from a CO, detect whether the downstreamDSL signal is corrupted by non-crosstalk noise, send an error feedbacksignal to the CO that indicates a measured error due to crosstalk noisein the downstream DSL signal if the downstream DSL signal is notsubstantially corrupted by non-crosstalk noise, and send a specialfeedback signal to the CO that indicates that non-crosstalk noise hasbeen detected instead of the measured error in the downstream DSL signalif the downstream DSL signal is substantially corrupted by non-crosstalknoise.

In yet another embodiment, the disclosure includes a method comprisingobtaining an error sample for a tone in a received symbol from a CO,detecting that the tone is corrupted if the error sample is corrupteddue to impulse noise or clipping or is otherwise unreliable, and sendingan error vector that comprises a special predefined value for the errorsample to a VCE coupled to a crosstalk precoder to indicate to the VCEthat the error sample and the tone are corrupted, wherein the specialvalue for the error sample comprises a real component and an imaginarycomponent that each comprise the same quantity of bits L_(w), andwherein all the bits in the real component and in the imaginarycomponent are equal to one.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a DSL system.

FIG. 2 is a schematic diagram of an embodiment of a crosstalk trainingsystem.

FIG. 3 is a flowchart of an embodiment of a crosstalk training method.

FIG. 4 is a flowchart of another embodiment of a crosstalk trainingmethod.

FIG. 5 is a schematic diagram of one embodiment of a general-purposecomputer system.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

In some cases, a plurality of CPEs in a DSL system may receive asequence of symbols from the CO that is corrupted by relatively largenoise bursts, for example due to impulse noise. The impulse noise may becharacterized by several peaks or bursts of relatively high levels andshort time intervals. Due to the impulse noise, the CPEs may notaccurately measure the errors in the tones that reflect the properamount of crosstalk noise, and therefore send inappropriate andmisleading error feedback signals to the CO. The error feedback signalsmay be received by a VCE in the CO, which may provide inappropriateprecoding coefficients based on the inappropriate error feedbacksignals. The inappropriate precoding coefficients may be used by acrosstalk precoder in the CO to add incorrect pre-distortions tosubsequent symbols from the CO to the CPEs. The CPEs may also receivesymbols that comprise substantially large signal levels that exceed thedynamic range of the system receivers, e.g. due to time varying radiofrequency interference (RFI) or out-of-domain crosstalk. Due to thelimited dynamic range of the receivers, the substantially large signallevels may be clipped. In this case, the CPEs may also fail toaccurately measure the errors in the tones of the symbols that reflectthe proper amount of crosstalk noise and may send incorrect errorfeedback signals to the CO. Such may cause the crosstalk precoder to addincorrect pre-distortions in subsequent symbols from the CO to the CPEs.The incorrect pre-distortions in the signals transmitted from CO may notproperly compensate for the crosstalk noise and consequently may disruptthe crosstalk training/tracking process, slow down the training time,degrade system performance in terms of achievable data-rates, orcombinations thereof.

Disclosed herein is a system and method for providing improved errorfeedback signals from the CPEs to avoid adding incorrect pre-distortionsin subsequent symbols by the crosstalk precoder, e.g. due to impulsenoise, RFI, out-of-domain crosstalk, and/or other non-crosstalk noisesources. Accordingly, a VTU-R at the CPE may comprise an impulse noisemonitor that monitors the received downstream signals from the CO forimpulse noise and/or other non-crosstalk noise. If the impulse noisemonitor detects impulse noise and/or other non-crosstalk noise in thesymbols received by the VTU-R, the VTU-R may send a special or reservederror signal to the VCE to prevent the crosstalk precoder from updatingits coefficients using incorrect error signals and hence avoiddisrupting the crosstalk training/tracking process. Subsequently, if theimpulse noise monitor does not detect more impulse noise and/ornon-crosstalk noise in the received symbols, the VTU-R may resumesending error feedback signals that reflect proper amount of crosstalknoise to the VCE and the crosstalk precoder training/tracking may beresumed.

FIG. 1 illustrates one embodiment of a DSL system 100. The DSL system100 may be a VDSL or VDSL2 system, an asymmetric DSL (ADSL) or ADSL2system, or any other DSL system. The DSL system 100 may comprise aDigital Subscriber Line Access Multiplexer (DSLAM) 102 at the CO sideand a plurality of CPEs 104, which may be coupled to the DSLAM 102 via aplurality of subscriber lines 106. Some of the subscriber lines 106 maybe bundled in a binder 107. The DSLAM 102 may comprise a crosstalkprecoder 108, which may be coupled to a plurality of subscriber lines106. Additionally, the DSL system 200 may comprise a VCE 109 that may becoupled to the crosstalk precoder 108 and the CPEs 104 via a pluralityof feedback channels 113. The feedback channels 113 between the CPEs 104and the VCE 109 (shown in dotted lines) may correspond to upstreamlogical data paths from the CPEs 104 to the DSLAM 102 and may not bephysically separated from the subscriber lines 106 (shown in solidlines). The CPEs 104 may transmit the error feedback signals in thefeedback channels 113 through the subscriber lines 106 to a plurality ofcorresponding receivers in the DSLAM 102, which may then extract theerror feedback signals from the upstream data stream and send the errorfeedback signals to VCE 109. Additionally, the DSLAM system 102 mayoptionally comprise a network management system (NMS) 110 and a publicswitched telephone network (PSTN) 112. In other embodiments, the DSLAMsystem 102 may be modified to include splitters, filters, managemententities, and various other hardware, software, and functionality. TheNMS 110 may be a network management infrastructure that processes dataexchanged with the DSLAM 102 and may be coupled to one or more broadbandnetworks, such as the Internet. The PSTN 112 may be a network thatgenerates, processes, and receives voice or other voice-band signals.

The DSLAM 102 may be located at the CO side of the DSL system 100 andmay comprise switches and/or splitters, which may couple the NMS 110,the PSTN 112, and the subscriber lines 106. For instance, the splittermay be a 2:1 coupler that forwards data signals received from thesubscriber lines 106 to the NMS 110 and the PSTN 112, and forwards datasignals received from the NMS 110 and the PSTN 112 to the subscriberlines 106. Further, the splitter may optionally comprise one or morefilters to help direct data signals between the NMS 110, the PSTN 112,and the subscriber lines 106. Additionally, the DSLAM 102 may compriseat least one DSL transmitter/receiver (transceiver), e.g. a VTU-O, whichmay exchange signals between the NMS 110, the PSTN 112, and thesubscriber lines 106. The signals may be received and transmitted usingthe DSL transceiver, such as a modem.

The DSL transceiver or VTU-O of the DSLAM 102 may comprise a forwarderror correction (FEC) codeword generator that generates FEC data. TheDSL transceiver may also comprise an interleaver that interleaves thetransmitted data across a plurality of tones in a group of symbols. Forinstance, the DSL transceiver may use a discrete multi-tone (DMT) linecode that allocates a plurality of bits for each sub-carrier or tone ineach symbol. The DMT may be adjusted to various channel conditions thatmay occur at each end of a subscriber line. In an embodiment, the DSLtransceiver of the DSLAM 102 may be configured to transmit data atsimilar or different rates for each subscriber line 106.

The CPEs 104 may be located at the customer premises, where at leastsome of the CPEs 104 may be coupled to a telephone 114 and/or a computer116. The telephone 114 may be hardware, software, firmware, orcombinations thereof that generates, processes, and receives voice orother voice-band signals. The CPE 104 may comprise a switch and/or asplitter, which may couple the subscriber lines 106 and the telephone114 and the computer 116. The CPE 104 may also comprise a DSLtransceiver, e.g. a VTU-R, to exchange data between the CPE 104 and theDSLAM 102 via the subscriber line 106. For instance, the splitter may bea 2:1 coupler that forwards data signals received from the subscriberline 106 to the telephone 114 and the DSL transceiver, and forwardsvoice signals received from the telephone 114 and data signals from theDSL transceiver to the subscriber line 106. The splitter may optionallycomprise one or more filters to help direct signals to and from thetelephone 114 and the DSL transceiver.

The DSL transceiver or VTU-R of the CPE 104, e.g. a modem, may transmitand receive signals through the subscriber lines 106. For instance, theDSL transceiver may process the received signals to obtain thetransmitted data from the DSLAM 102, and pass the received signals tothe telephone 114, the computer 116, or both. The CPEs 104 may becoupled to the DSLAM 102 directly via the subscriber lines. For exampleany of the CPEs 104 may be coupled to a subscriber line 106 from theDSLAM 102. The CPEs 104 may access the NMS 110, the PSTN 112, and/orother coupled networks via the subscriber lines 106 deployed by theDSLAM 102.

The subscriber lines 106 may be telecommunications paths between theDSLAM 102 and the CPE 104, and may comprise one or more twisted-pairs ofcopper cable. Crosstalk interference may occur between a plurality ofsubscriber lines 106 that are deployed by the DSLAM 102, e.g. in thebinder 107. The crosstalk interference may be related to the power,frequency, and travel distance of the transmitted signals and may limitthe communications performance in the network. For instance, when thepower spectral density (PSD) of the transmitted signals increase, e.g.over a range of frequencies, the crosstalk between the adjacentsubscriber lines 106 may increase and hence the data rates may decrease.The propagation of the signals in the downstream direction from theDSLAM 102 to the CPEs 104 may be represented by:y=Hx+z,  (1)where y is a vector that represents the signals at the CPEs 104, H is amatrix that represents the crosstalk channels in the lines, x is avector that represents the signals from the DSLAM 102, and z is a vectorthat represents random errors or noise.

The crosstalk precoder 108 may be configured to reduce or limit thecrosstalk in the lines. The crosstalk precoder 108 may transmitpre-distorted downstream signals in the subscriber lines 106 to cancelor reduce crosstalk error in the lines. The crosstalk precoder 108 mayprocess a plurality of downstream signals from the DSLAM 102 transmitter(e.g. from a plurality of VTU-Os), add distortion to the downstreamsignals, and transmit the pre-distorted downstream signals to the CPEs104 via the subscriber lines 106. The pre-distorted signals may begenerated by the crosstalk precoder 108 whose parameters are properlychosen to minimize the crosstalk in the downstream channels. In orderfor the crosstalk precoder to select the appropriate parameters, theCPEs 104 may send back the error signals in the downstream receivers asfeedback for the precoder 108 to update its parameters. For instance, aplurality of VTU-Rs at the CPEs 104 may measure the errors for aplurality of received symbols (e.g. DMT symbols) from the crosstalkprecoder 108, and transmit back to the VCE 109 a plurality ofcorresponding error feedback signals, via a feedback channel.

Typically, the feedback channel 113 may be established through upstreamdata signal paths from the CPEs 104 to the DSLAM 102, which may beprovided in addition to upstream communications data. The upstreamreceivers in the DSLAM 102 may isolate the error feedback signals fromthe upstream communications data and send the error feedback signals tothe VCE 109. The VCE 109 may be configured to control the crosstalkprecoder 108 to adapt the crosstalk precoder 108 based on the errorfeedback signals from the CPEs 104. Thus, the crosstalk precoder 108 maysend appropriate pre-distorted signals to the CPEs 104, which mayproperly cancel or substantially reduce the crosstalk in the downstreamsignals received at the CPEs 104. The VCE 109 may use the error feedbacksignals from the VTU-Rs at the CPEs 104 to identify the crosstalkchannels in the lines, calculate precoding coefficients, and update aprecoding matrix for the crosstalk precoder 108. The precoding matrixmay comprise the precoding coefficients, which may be calculated basedon an adaptive algorithm, such as a least mean square (LMS) algorithm ora recursive least square (RLS) algorithm, or other proper algorithms.The crosstalk precoder 108 may use the precoding coefficients and matrixto produce the pre-distorted signals for the lines. Cancelling thecrosstalk using signal distortion may be represented by:

$\begin{matrix}\begin{matrix}{y = {{HPx} + z}} \\{{= {{{diag}\left\{ H \right\} x} + z}},}\end{matrix} & (2)\end{matrix}$where P=H⁻¹ diag{H} and is a precoding matrix configured to cancel orsubstantially eliminate the crosstalk channels in the lines.

The process of sending the symbols to the VTU-Rs and receivingcorresponding error feedback signals may be repeated over multipleperiods of time during downstream transmissions to improve the output ofthe crosstalk precoder 108, and hence improve crosstalk cancelation.Such periods of time may be referred to as the training orinitialization time of the crosstalk precoder 108. During training time,a sequence of pilot symbols may be transmitted and accordingly asequence of error feedback signals may be received (e.g. for eachsubscriber line 106) until the pre-distorted pilot symbols from thecrosstalk precoder 108 converge to a pattern or value. Afterinitialization, the CPEs 104 may continue to calculate or measure theerror signals in the received downstream data symbols and send backerror feedback signals to the DSLAM 102, which may then forward theerror feedback signals to the VCE 109 to continue updating the precodingcoefficients to track the crosstalk channel variations.

In some cases, the CPE 104 may receive downstream symbols that arecorrupted by impulse noise, RFI noise, out-of-domain crosstalk noise,other non-crosstalk noise, or combinations thereof. In such cases, theCPE 104 may incorrectly calculate or measure the errors corresponding tocrosstalk noise in the received signals. For instance, the VTU-R (e.g.the slicer) at the CPE 104 may incorrectly demap a received QuadratureAmplitude Modulated (QAM) symbol that comprises impulse noise,out-of-range signals, and/or clipped signals. Further, some signals maycomprise signal values that may be outside the dynamic range of thereceiver and hence may be distorted by clipping. As such, the VTU-R maysend incorrect error feedback signals to the VCE 109, and consequentlythe crosstalk precoder 108 may be updated using the incorrect errorfeedback signals, and add incorrect pre-distortions in the subsequentlytransmitted symbols based on the improper precoding coefficients due toincorrect error feedback signals.

To avoid the disruptive effects of impulse noise, out-of-range signals,and/or clipped signals on the crosstalk training process, the VTU-R atthe CPE 104 may send a special or reserved error signal to the VCE 109in response to detecting impulse noise, out-of-range signals, and/orclipped signals. When the reserved signal is received by the VCE 109,the crosstalk training process may be paused, e.g. until a subsequentproper error feedback signal that properly represents crosstalk noise isreceived from the VTU-R. Additionally or alternatively, the VTU-R maysend an error feedback signal that comprises a flag to indicate to theVCE 109 that impulse noise, an out-of-range signal, and/or a clippedsignal has been detected at the CPE 104.

FIG. 2 illustrates an embodiment of a crosstalk training system 200,which may be used in the DSL system 100 to cancel or substantiallyreduce crosstalk. Additionally, the crosstalk training system 200 mayaccount for impulse noise, out-of-range signals, and/or clipped signalsto improve the crosstalk training process, e.g. avoid training time slowdown and/or data-rate reduction. The crosstalk training system 200 maycomprise a VTU-R 204 that comprises an impulse noise monitor 205, acrosstalk precoder 208 coupled to the VTU-R 204, and a VCE 209 coupledto the VTU-R 204 and the crosstalk precoder 208. The VTU-R 204 and theVCE 209 may be coupled via the upstream data paths from the VTU-R 204 toa VTU-O (not shown) at the DSLAM (e.g. DSLAM 102). The components of thecrosstalk training system 200 may be substantially similar to thecorresponding components of the DSL system 100. The VTU-R 204 may belocated at a CPE, e.g. the CPE 104, and may communicate with acorresponding VTU-O (not shown) at the CO, e.g. at the DSLAM 102. TheVTU-O may be coupled to the crosstalk precoder 208 and the VCE 209,which may also be located at the CO.

The impulse noise monitor 205 may be configured to monitor the receivedsignals from the VTU-O and to detect the presence of any significantimpulse noise in the signals. For instance, the impulse noise monitor205 may detect a plurality of signal peaks or bursts that correspond toimpulse noise levels. The signal peaks or bursts may be above theexpected DSL signal levels and may be detected in the received signalsover periods of time. For example, the signal bursts may be detectedover a plurality of received DMT symbols that may exceed the expecteddownstream signal levels. The impulse noise monitor 205 may also detectthe burst of relatively large error signals in a plurality of tones,which may indicate the presence of relatively strong impulse noise.

The VTU-R 204 may be configured to send a special or reserved errorsignal to the VCE 209 when the impulse noise monitor 205 detects impulsenoise, out-of-range signals, and/or clipped signals. The VTU-R 204 maysend the special or reserved error signal to the VCE 209 instead ofincorrect error feedback signals due to impulse noise or othernon-crosstalk noise to prevent the updating the crosstalk precoder 208with incorrect error feedback and thus generating incorrectpre-distorted downstream signals to the VTU-R 204 based on improperprecoding coefficients. For instance, the VTU-R 204 may set the errorfeedback signal to a value that indicates that impulse noise,out-of-range signals, and/or clipped signals were detected. The VCE 209may receive the reserved error feedback signal from the VTU-R 204, andhence pause the crosstalk training process, e.g. until proper errorfeedback signals are subsequently sent from the VTU-R 204. Specifically,upon receiving the reserved error feedback signal, the VCE 209 may notupdate the precoding coefficients for the crosstalk precoder 208 andthus prevent the crosstalk precoder 208 from drifting away fromappropriate coefficient values and thus adding inappropriatepre-distortions to the downstream signals from the VTU-O.

The VTU-R 204 may set the error feedback signal to a sequence of allzeros when the impulse noise monitor 205 detects impulse noise,out-of-range signals, and/or clipped signals. The VCE 209 may update theprecoding coefficients and matrix for the crosstalk precoder 208 usingthe zero sequence, which may not change the coefficients of thecrosstalk precoder 208. This may prevent the divergence of precodingcoefficients due to impulse noise. However, if the VTU-R 204 continuesto send the sequence of zeros to the VCE 209, e.g. as a result offurther detecting impulse noise and/or other non-crosstalk noise, theVCE 209 may end the crosstalk training process prematurely beforeproperly evaluating and compensating for the crosstalk noise levels inthe system.

To avoid ending the crosstalk training process prematurely, the VTU-R204 may alternatively set the error feedback signal to a non-zeroreserved sequence, e.g. of a sequence of all ones, or other reservedspecial value, to indicate to the VCE 209 that impulse noise or othernon-crosstalk noise was detected. When the VCE 209 receives the reservedsequence, the VCE 209 may become aware of impulse noise and/or othernon-crosstalk noise in the downstream signals at the VTU-R 204 noise andconsequently may not update the precoding coefficients for the crosstalkprecoder 208. The VCE 209 may also be aware that the pause in thecoefficients update is temporary due to the presence of impulse noise,and hence may wait for subsequent correct error feedback signals fromthe VTU-R 204 that reflect proper crosstalk noise in the system.

After sending the reserved error feedback signal to the VCE 209 inresponse to detecting the impulse noise and/or other non-crosstalknoise, the VTU-R 204 may resume receiving downstream signals that arenot corrupted by impulse noise or other non-crosstalk noise. Thus, theVTU-R 204 may resume sending error feedback signals to the VCE 209,which may properly represent crosstalk noise in the received signals.Such error feedback signals may be different than the special orreserved error feedback signal, e.g. the non-zero sequence or thesequence of all ones. Consequently, the VCE 209 may resume updating theprecoding coefficients and the crosstalk precoder 208 may add properpre-distortions in the downstream signals. Thus, the crosstalktraining/tracking process may continue and the crosstalk precoder 208may reach the optimal coefficients that provide optimal pre-distorteddownstream signals to achieve optimal crosstalk cancellation.

In some embodiments, the VTU-R 204 may send the same reserved errorfeedback signal or different reserved error feedback signals to the VCE209, which may indicate different non-crosstalk noise sources in thereceived downstream signals, e.g. impulse noise, out-of-range signals,and/or clipped signals. For example, the VTU-R 204 may send a firstreserved error feedback value that indicates detecting impulse noisebursts in the received signals and at least a second reserved errorfeedback signal that indicates detecting other non-crosstalk noisesignals in the received signal.

In an embodiment, the VTU-R 204 may send a special or reserved value inthe error feedback signal to the VCE 209 that indicates that a tone inthe received symbol is corrupted due to impulse noise, RFI,out-of-domain crosstalk, or other non-crosstalk noise source. The errorfeedback signal that corresponds to a symbol may comprise a plurality oferror vectors that corresponds to a plurality of tones in the receivedpilot symbol. The error vectors may indicate the measured errors in thereceived tones, for example a plurality of normalized errors in thereceived tones. The VTU-R 204 may set all the bits in the error vectorthat correspond to a tone in the pilot symbol to all ones to indicatethat it has detected a corrupted tone due to non-crosstalk noise. Theerror signal for the tone may comprise a real component and an imaginarycomponent, which may be both set to all ones when the VTU-R 204 detectsimpulse noise and/or other non-crosstalk noise in the tone. Further, ifa substantial quantity of corrupted tones are detected in the receivedpilot symbol, such as at least half (or a specified threshold) of thetones in the symbol, the entire symbol may be unsuitable to evaluatecrosstalk noise. As such, the measured errors for all the tones (e.g.normalized errors) may be discarded and the VTU-R 204 may set the bitsof the real and imaginary components of all the error vectors in theerror feedback signal, which correspond to all the tones in the symbol,to all ones.

FIG. 3 illustrates an embodiment of a crosstalk training method 300,which may be used to improve crosstalk training in a DSL system, e.g.the DSL system 100, and account for impulse noise and/or other noisesources. For instance, the crosstalk training method 300 may beimplemented by the VTU-R 204 and the impulse noise monitor 205 in thecrosstalk training system 200. The method 300 may begin at block 310,where a downstream signal may be received. For example, the VTU-R 204 atthe CPE 104 may receive a tone in a DMT symbol from the VTU-O at thecentral office. At block 320, an error in the received downstream signalmay be measured. The error may be caused by crosstalk interference,impulse noise, and/or other noise sources. For example, the VTU-R 204may measure the error value in the received signal based on the expectedsignal value of the tone in the symbol. At block 330, the method 300 maydetermine whether the measured error is corrupted by impulse noiseand/or other non-crosstalk noise source. For example, the impulse noisemonitor 205 may process the measured error and/or received signal todetect any relatively large bursts due to impulse noise, anysubstantially high levels that exceed the dynamic range, any relativelylarge error values, and/or error due to RFI. The method 300 may proceedto block 340 if the condition in block 330 is met. Otherwise, the method300 may proceed to block 345.

At block 340, an error feedback signal that corresponds to the measurederror may be set to a special or reserved value. For example, the VTU-R204 may set the bits in the error feedback signal that corresponds tothe corrupted tone to all ones. In another embodiment, a flag thatcorresponds to the measured error or tone may be set in the errorfeedback signal to indicate that a non-crosstalk noise source has beendetected in the downstream signal at the CPE. Alternatively, at block345 the error feedback signal may be set to the measured error value,e.g. due to crosstalk noise. At block 350, the error feedback signal maybe sent over the feedback channel, e.g. to the VCE 209. At block 360,the method 300 may determine whether there are more downstream signalsto be received. The method 300 may return to block 310 if the receptionof downstream signal continues. Otherwise, the method 300 may end.

FIG. 4 illustrates another embodiment of a crosstalk training method400, which may be used to improve crosstalk training in a DSL system,e.g. the DSL system 100, and account for impulse noise and/or othernoise sources. For instance, the crosstalk training method 300 may beimplemented by the VCE 209 and the crosstalk precoder 208 in thecrosstalk training system 200. The method 400 may begin at block 410,where an error feedback signal may be received. For example, the VCE 209at the central office may receive an error feedback signal from theVTU-R 204 via the feedback channel, which may be through the upstreamdata path. The error feedback signal may comprise a plurality of errorvectors that represent a plurality of measured errors for a plurality oftones in a downstream symbol received at the VTU-R 204. At block 420,the method 400 may determine whether the received error feedback signalcomprises a special or reserved value that indicates detecting impulsenoise and/or other non-crosstalk noise. For example, the VCE 209 maycheck whether any error vector in the error feedback signal comprises aspecial or reserve sequence, e.g. a sequence of all ones, which may beset by the VTU-R 204 due to detecting a corrupted tone. In anotherembodiment, the VCE 209 may check whether a reserved flag in the errorfeedback signal that corresponds to the tone in the symbol is set toindicate that a significant or substantial non-crosstalk noise sourcewas detected at the VTU-R. The method 400 may proceed to block 430 ifthe condition in block 420 is met. Otherwise, the method 400 may proceedto block 435.

At block 430, the precoder coefficient update is paused. For instance,the VCE 209 may not update the precoder coefficients for the crosstalkprecoder 208, and thus the last updated precoder coefficients 208 may bestill be used to generate pre-distortion to the next transmitteddownstream signal from the VTU-O to the VTU-R 204. Alternatively, atblock 435 the precoder coefficients may be updated based on the receivederror feedback signal. For instance, the VCE 209 may update the precodercoefficients using the error feedback signal from the VTU-R 204, andthus the crosstalk precoder 208 may add better pre-distortion to thenext transmitted downstream signal according to the updated precodercoefficients. At block 440, the precoder coefficients may be applied togenerate and transmit a downstream signal, e.g. from the VTU-O to theVTU-R 204. At block 450, the method 400 may determine whether tocontinue (or repeat), e.g. to process a second error feedback signal, orto end. If a second feedback signal was transmitted, the method 400 mayreturn to block 410. Otherwise, the method 400 may end.

The components described above may be operated in conjunction with anygeneral-purpose network component, such as a computer or networkcomponent with sufficient processing power, memory resources, andnetwork throughput capability to handle the necessary workload placedupon it. FIG. 5 illustrates a typical, general-purpose network component500 suitable for implementing one or more embodiments of the componentsdisclosed herein. The network component 500 may include a processor 502(which may be referred to as a central processor unit or CPU) that is incommunication with any memory devices including secondary storage 504,read only memory (ROM) 506, random access memory (RAM) 508, input/output(I/O) devices 510, and network connectivity devices 512, or combinationsthereof. The processor 502 may be implemented as one or more CPU chips,or may be part of one or more application specific integrated circuits(ASICs).

The secondary storage 504 is typically comprised of one or more diskdrives or other storage devices and is used for non-volatile storage ofdata and as an over-flow data storage device if RAM 508 is not largeenough to hold all working data. Secondary storage 504 may be used tostore programs that are loaded into RAM 508 when such programs areselected for execution. The ROM 506 is used to store instructions andperhaps data that are read during program execution. ROM 506 is anon-volatile memory device that typically has a small memory capacityrelative to the larger memory capacity of secondary storage 504. The RAM508 is used to store volatile data and perhaps to store instructions.Access to both ROM 506 and RAM 508 is typically faster than to secondarystorage 504.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations. For example, whenever a numerical range with alower limit, R_(l), and an upper limit, R_(u), is disclosed, any numberfalling within the range is specifically disclosed. In particular, thefollowing numbers within the range are specifically disclosed:R=R_(l)+k*(R_(u)−R_(l)), wherein k is a variable ranging from 1 percentto 100 percent with a 1 percent increment, i.e., k is 1 percent, 2percent, 3 percent, 4 percent, 5 percent, . . . , 50 percent, 51percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98percent, 99 percent, or 100 percent. Moreover, any numerical rangedefined by two R numbers as defined in the above is also specificallydisclosed. Use of the term “optionally” with respect to any element of aclaim means that the element is required, or alternatively, the elementis not required, both alternatives being within the scope of the claim.Use of broader terms such as comprises, includes, and having should beunderstood to provide support for narrower terms such as consisting of,consisting essentially of, and comprised substantially of Accordingly,the scope of protection is not limited by the description set out abovebut is defined by the claims that follow, that scope including allequivalents of the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare embodiment(s) of the present disclosure. The discussion of areference in the disclosure is not an admission that it is prior art,especially any reference that has a publication date after the prioritydate of this application. The disclosure of all patents, patentapplications, and publications cited in the disclosure are herebyincorporated by reference, to the extent that they provide exemplary,procedural, or other details supplementary to the disclosure.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a first transceiver at acentral office (CO) configured to transmit a downstream signal to asecond transceiver at a customer premise equipment (CPE) via a digitalsubscriber line (DSL); a crosstalk precoder coupled to the firsttransceiver at the CO; and a vectoring control entity (VCE) coupled tothe crosstalk precoder to control the crosstalk precoder, wherein thefirst transceiver is further configured to receive a first errorfeedback signal from the second transceiver, wherein the first errorfeedback signal comprises a fixed value that indicates that content ofthe first error feedback signal is potentially corrupted bynon-crosstalk noise, and wherein the VCE takes responsibility torestrict the crosstalk precoder from updating a precoding coefficientthat corresponds to the first error feedback signal when the first errorfeedback signal comprises the fixed value that indicates that thecontent of the first error feedback signal is potentially corrupted bythe non-crosstalk noise.
 2. The apparatus of claim 1, wherein the firsterror feedback signal comprises a flag that is set to the fixed valuethat indicates that the content of the first error feedback signal ispotentially corrupted by the non-crosstalk noise.
 3. The apparatus ofclaim 2, wherein the first error feedback signal further comprises anerror vector that represents a measured error for a tone in thedownstream signal, and wherein the error vector comprises a realcomponent that comprises a quantity of bits and an imaginary componentthat comprises a quantity of bits.
 4. The apparatus of claim 1, whereinthe fixed value is a non-zero value that is different than a measurederror.
 5. The apparatus of claim 1, wherein the first transceiver isfurther configured to receive a second error feedback signal from thesecond transceiver, wherein the second error feedback signal comprises aplurality of error vectors that represent a plurality of measured errorsfor a plurality of tones in the downstream signal, and wherein each ofthe error vectors comprises a real component that comprises a quantityof bits and an imaginary component that comprises a quantity of bits. 6.The apparatus of claim 1, wherein the non-crosstalk noise corresponds toimpulse noise in the downstream signal.
 7. The apparatus of claim 1,wherein the non-crosstalk noise corresponds to radio frequencyinterference (RFI) noise or out-of-domain crosstalk noise in thedownstream signal.
 8. The apparatus of claim 1, wherein the downstreamsignal is distorted by clipping due to abnormally high signal levels,and wherein signal clipping results in incorrect measured error values.9. An apparatus comprising: a first transceiver at a customer premiseequipment (CPE) configured to receive downstream digital subscriber line(DSL) signals from a second transceiver at a central office (CO) via aDSL; and a monitor coupled to the first transceiver and configured todetect whether the downstream DSL signals are corrupted by non-crosstalknoise, wherein the first transceiver is further configured to send afirst error feedback signal to a vectoring control entity (VCE) coupledto the second transceiver, wherein the first error feedback signalcomprises a fixed value that indicates that content of the first errorfeedback signal is potentially corrupted by non-crosstalk noise; andwherein the fixed value is received by the VCE and instructs the VCE torestrict a crosstalk precoder from updating a precoder coefficient thatcorresponds to the first error feedback signal comprising the fixedvalue.
 10. The apparatus of claim 9, wherein the first error feedbacksignal comprises a flag that is set to the fixed value that indicatesthat the content of the first error feedback signal is potentiallycorrupted by the non-crosstalk noise.
 11. The apparatus of claim 10,wherein the fixed value is a non-zero value that is different than ameasured error.
 12. The apparatus of claim 9, wherein the firsttransceiver is further configured to send a second error feedback signalto the VCE, wherein the second error feedback signal comprises aplurality of error vectors that represent a plurality of measured errorsfor a plurality of tones in the downstream DSL signals, and wherein eachof the error vectors comprises a real component that comprises aquantity of bits and an imaginary component that comprises a quantity ofbits.
 13. The apparatus of claim 9, wherein the non-crosstalk noisecomprises impulse noise, radio frequency interference (RFI) noise,out-of-range errors, clipped signal errors, or combinations thereof. 14.An apparatus comprising a first transceiver at a customer premiseequipment (CPE), configured to receive a discrete multi-tone (DMT)symbol from a second transceiver at a central office (CO), wherein thefirst transceiver is further configured to: obtain a measured error fora plurality of tones in the received DMT symbol from the secondtransceiver; detect that a first tone in the received DMT symbol iscorrupted due to non-crosstalk noise comprising at least one of impulsenoise and radio frequency interference (RFI) noise; and send a firsterror feedback signal corresponding to the first tone of the tones to avectoring control entity (VCE) coupled to the second transceiver,wherein the first error feedback signal corresponding to the first tonecomprises a fixed value that indicates that content of the first errorfeedback signal is potentially corrupted by non-crosstalk noise, andwherein the VCE takes responsibility to restrict updating a crosstalkprecoder coefficient of a crosstalk precoder coupled to the secondtransceiver in response to a reception of the first error feedbacksignal comprising the fixed value.
 15. The apparatus of claim 14,wherein the first error feedback signal comprises a flag that is set tothe fixed value that indicates that the content of the first errorfeedback signal is potentially corrupted by the non-crosstalk noise. 16.The apparatus of claim 14, wherein the first transceiver is furtherconfigured to send a second error feedback signal corresponding to asecond tone in the received DMT symbol to the VCE when detecting thatthe second tone is not corrupted due to the non-crosstalk noise, whereinthe second error feedback signal comprises an error vector thatrepresents a measured error for the second tone in the received DMTsymbol, and wherein the error vector comprises a real component having aquantity of bits and an imaginary component having a quantity of bits.